Integration of optical components on a single chip is a desired feature. Integration reduces the number of components, reduces the device size and eliminates all the fiber interconnection. Thus the device reliability is increased, the performance is improved and the overall cost is significantly reduced.
The Planar Lightwave Circuit (PLC) SiO2 on Si technology is a natural and mature technology for integration for the following reasons: (1) many components such as AWGs (Array Waveguide Gratings), switches, VOAs (Variable Optical Attenuator), splitters, taps, etc, have already been produced with this technology, and (2) PLC technology uses almost the same equipment and processes as used by the mature microelectronic industry. Some PLC integrated components have already been fabricated, most of them in the configuration of the basic OADM (Optical Add Drop Multiplexer) module [T. Saida, A. Kaneko, T. Goh, M. Okuno, A. Himeno, K. Takiguchi, K. Okamoto, xe2x80x9cAthermal silica-based optical add/drop multiplexer consisting of arrayed waveguide gratings and double gate thermo-optical switches,xe2x80x9d Elect. Lett., 36, 528-529, 2000], shown in FIG. 1. FIG. 1 shows an integrated PLC OADM module 10 with an AWG multiplexer (xe2x80x9cpassivexe2x80x9d component) 12, N xe2x80x9cadd/dropxe2x80x9d 2xc3x972 switches 14 (xe2x80x9cactivexe2x80x9d components) and an AWG multiplexer 16 (xe2x80x9cpassivexe2x80x9d component). A major problem in this integration lies in preventing the active (heat producing) components from affecting the performance of the (temperature) sensitive passive components. A change in the configuration of a thermo-optic switch matrix is usually followed by a variation in the local distribution of the operating heaters, as shown in U.S. Pat. No. 6,259,834 to Shani. There, the distribution of the operating (xe2x80x9conxe2x80x9d stage) switches depends on the switch matrix configuration, and switches that change from a heated to a non-heated state induce local temperature non-uniformities on the wafer. Thus, even if the whole wafer is temperature-stabilized, changing a switch matrix configuration can affect the integrated passive components performance.
Prior art methods for removing the temperature sensitivity include: (1) fabricating active and passive components on separate chips and butt-coupling them; (2) separating between the active and passive components which are integrated on the same chip, and (3) designing the passive components to be a-thermal components [e.g. M. Ishii, Y. Hibino, F. Hanawa, H. Nakagome, K. Kato, xe2x80x9cPackaging and environmental stability of thermally controlled AWG multiplexer module with thermoelectric device,xe2x80x9d J. Light. Technology, vol 6, 258-264, 1998; N. Kail, H. H. Yao, C. Zawadzki, xe2x80x9cAthermal polarization-independent AWG multiplexer using an all polymer approach,xe2x80x9d European Conference on Integrated Optics, Paderborn, Apr. 4-6 2001, Post-deadline paper]. However, these prior art methods suffer from a number of drawbacks and disadvantages, including the use of non-mature polymer technology, and hybrid integration of half waveplates.
The prior art operation and use of thermo-optic switches is illustrated next. FIG. 2 shows a common Mach Zehnder Interferometer (MZI) thermo-optic switch 100. Such switches are known in the art, and a detailed description of one is provided for example in M. Kawachi, xe2x80x9cSilica waveguides on silicon and their application to integrated-optic components,xe2x80x9d Optical and Quantum Electronics, 417-426, 1990 (hereinbelow KAW90). Switch 100 consists of two 3 dB couplers 102 and 104 combined by two waveguide arms 106 and 108, with an electrode (xe2x80x9cheaterxe2x80x9d) 110 on one of the waveguide arms (in this case, arm 106). In the xe2x80x9coffxe2x80x9d position or stage, electrode 110 is not activated (not turned on) and therefore introduces no phase difference, and the light passes from an input 112 on one arm (e.g. arm 106) to an output 114 on the other arm (108), i.e. following the xe2x80x9c1xe2x80x9d greater than xe2x80x9c2xe2x80x9d path in FIG. 2. In the xe2x80x9conxe2x80x9d position electrode 110 is activated, a 180 degree phase shift due to the thermo-optic effect is introduced by the electrode on the light passing in arm 106, and the light stays in the input waveguide (arm 106) leading to an output 116, i.e. following the xe2x80x9c1xe2x80x9d greater than xe2x80x9c1xe2x80x9d path. The action of the switch is xe2x80x9cdigitalxe2x80x9d in the sense that it operates in two positions only, one position requiring heating, the other not requiring it. The passage from a heated to a non-heated state is the main source of temperature non-uniformity on the wafer or chip.
In order to build a constant power operating switch, i.e. to have a constant heating power during both the xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d stages (and therefore remove the temperature non-uniformity), a 90 degree phase shift is added between the waveguide arms, and electrodes 110 and 120 are positioned on both MZI arms (106 and 108 respectively), as shown in FIG. 3. In this case, with no heating (xe2x80x9czero power consumptionxe2x80x9d) no additional phase shift is introduced and the light goes to both output arms (114 and 116), i.e. the switch functions as a 3 db splitter. This is the main use of this architecture, as described in both U.S. Pat. No. 6,259,834 to Shani and in KAW90. For a xe2x80x9cswitchxe2x80x9d operation, path xe2x80x9c1xe2x80x9d greater than xe2x80x9c1xe2x80x9d is connected when heater 110 is turned xe2x80x9conxe2x80x9d (and heater 120 is xe2x80x9coffxe2x80x9d), and path xe2x80x9c1xe2x80x9d greater than xe2x80x9c2xe2x80x9d is connected when heater 120 is turned xe2x80x9conxe2x80x9d (and heater 110 is xe2x80x9coffxe2x80x9d). Thus, in this xe2x80x9cswitchxe2x80x9d or xe2x80x9cdigitalxe2x80x9d mode, there is always one heater in an xe2x80x9conxe2x80x9d position (and one heater in an xe2x80x9coffxe2x80x9d position), independently of whether the connection is xe2x80x9c1xe2x80x9d greater than xe2x80x9c1xe2x80x9d or xe2x80x9c1xe2x80x9d greater than xe2x80x9c2xe2x80x9d. However, if the device is used both as a splitter (both heaters xe2x80x9coffxe2x80x9d, zero power) and as a switch (one heater xe2x80x9conxe2x80x9d, the other xe2x80x9coffxe2x80x9d), there is still a non-uniform and time dependent temperature distribution on the chip.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method for operating such a switch that does not suffer from the disadvantages of prior art switches, and that provides a uniform temperature distribution on a PLC, thus not affecting the integrated passive components performance.
The present invention is of a method used to eliminate temperature variations caused by active components in PLCs. More specifically, the method of the present invention can be used to design an active component (e.g. switch) in such a way that its generated heat does not depend on its configuration (i.e. the same heat generation exists if the active component is at an xe2x80x9conxe2x80x9d or at an xe2x80x9coffxe2x80x9d position).
According to the present invention there is provided a method for obtaining a constant and uniform temperature on a planar lightwave circuit, comprising: a) providing at least one active element having two connecting configurations and operative to have a phase change in a light beam passing through each of the connecting configurations, and b) constantly heating both of the connecting configurations, thereby achieving a substantially constant and uniform temperature distribution on the planar lightwave circuit.
According to one feature of the method of the present invention for obtaining a constant and uniform temperature on a planar lightwave circuit, the step of providing at least one active element includes providing at least one thermo-optic switch.
According to another feature of the method of the present invention for obtaining a constant and uniform temperature on a planar lightwave circuit, the step of providing at least one thermo-optic switch includes providing a waveguide Mach Zehnder Interferometer switch having one input waveguide, two identical waveguide arms, and two output waveguides, wherein the connecting configurations include a first connecting configuration defined by connecting the input waveguide to one of the output waveguides, and a second connecting configuration defined by connecting the input waveguide to the other of the output waveguides.
According to yet another feature of the method of the present invention for obtaining a constant and uniform temperature on a planar lightwave circuit, the step of constantly heating both of said connecting configurations includes providing a heater connected to each of the waveguide arms, and simultaneously heating each of the heaters using a respective heating power P in order to achieve a desired power difference configuration xcex94P related to the phase change in each of the waveguide arms.
According to yet another feature of the method of the present invention for obtaining a constant and uniform temperature on a planar lightwave circuit, the substep of simultaneously heating using a respective heating power P includes using a power P1 for one of the heaters and a power P2 for the other of said heaters, wherein P1, P2 and xcex94P are expressed by equation 2.
According to yet another feature of the method of the present invention for obtaining a constant and uniform temperature on a planar lightwave circuit, P1 and P2 and xcex94P as expressed by equation 2 render the Mach Zehnder Interferometer switch operable in a digital mode.
According to yet another feature of the method of the present invention for obtaining a constant and uniform temperature on a planar lightwave circuit, P1 and P2 and xcex94P as expressed by equation 2 render the Mach Zehnder Interferometer switch operable in an analog mode.
According to yet another feature of the method of the present invention for obtaining a constant and uniform temperature on a planar lightwave circuit, the Mach Zehnder Interferometer switch is made of silica on a silicon substrate.
According to the present invention there is provided a method for operating a planar lightwave circuit at a constant power consumption, comprising a) providing a matrix of integrated active elements, b) providing a heating power to each active element to independently heat the active element, and c) cooperatively operating the heating powers of the active elements to keep a sum of the operating heating powers constant.
According to one feature of the method of the present invention for operating a planar lightwave circuit at a constant power consumption, each active element is a thermo-optic switch further characterized by having two connecting configurations and operative to have a phase change in a light beam passing through each of said connecting configurations.
According to another feature of the method of the present invention for operating a planar lightwave circuit at a constant power consumption, the step of providing integrated thermo-optic switches includes providing waveguide Mach Zehnder Interferometer switches, each such Mach Zehnder Interferometer switch having one input waveguide, two identical waveguide arms, and two output waveguides, wherein the connecting configurations include a first connecting configuration defined by connecting the input waveguide to one of the output waveguides, and a second connecting configuration defined by connecting the input waveguide to the other of the output waveguides.
According to yet another feature of the method of the present invention for operating a planar lightwave circuit at a constant power consumption, the step of providing a heating power of each Mach Zehnder Interferometer switch includes constantly heating both of the connecting configurations.
According to yet another feature of the method of the present invention for operating a planar lightwave circuit at a constant power consumption, the substep of constantly heating both of the connecting configurations further includes providing a heater connected to each of the waveguide arms, and simultaneously heating each of the heaters using a respective heating power P in order to achieve a desired power difference configuration xcex94P related to the phase change in each of the waveguide arms.
According to yet another feature of the method of the present invention for operating a planar lightwave circuit at a constant power consumption, the substep of simultaneously heating using a respective heating power P includes using a power P1 for one of the heaters and a power P2 for the other of the heaters, wherein P1, P2 and xcex94P are expressed by equation 2.
According to yet another feature of the method of the present invention for operating a planar lightwave circuit at a constant power consumption, P1, P2 and xcex94P as expressed by equation 2 render the Mach Zehnder Interferometer switch operable in a digital mode.
According to yet another feature of the method of the present invention for operating a planar lightwave circuit at a constant power consumption, P1, P2 and xcex94P as expressed by equation 2 render the Mach Zehnder Interferometer switch operable in an analog mode.
According to yet another feature of the method of the present invention for operating a planar lightwave circuit at a constant power consumption, the Mach Zehnder Interferometer switch is made of silica on a silicon substrate.